This invention pertains generally to antennas for radio frequency energy and particularly to array antennas for such energy.
Extensive work has been carried on for many years in attempts to enhance or reduce the radar cross-section (hereinafter sometimes referred to as "RCS") of different objects. In the course of such work it has been found that the monostatic and bistatic RCS of simple geometric shapes, as planar or spherical surfaces can be quantitatively evaluated to a great degree of accuracy. When the motion of the simple objects, with the exception of sphere, varies in a random and unpredictable manner, it has been customary for many system analysis purposes to considre the average RCS of such objects as the criterion for their RCS. When relatively complex objects, such as aircraft, are considered, the average RCS may not serve as an effective criterion because of the unpredictable variations in RCS as a function of variations of such object's aspect angle. When complex objects are considered, it is sometimes more convenient to use the peak, meaning the greatest, RCS as the criterion.
Whether one considers a complex object such as an aircraft in terms of its average or its peak RCS, it is evident that, if it is desired to reduce the probability of its detection by an interrogating radar, both the average and peak RCS of the aircraft and any equipment carried thereon which contributes to the RCS need be reduced. In the particular case in which a radar incorporating a phased array antenna is carried by an aircraft, an array may, at certain aspect angles, contribute significantly to the RCS.
Known ways of reducing RCS, as the employment of radio frequency absorbing materials or quarter-wave traps, cannot be used to any advantage with phased array antennas. The primary reason for the limited utility of such approaches is that known techniques may not be applied to phase array antennas without destroying their effectiveness. Further, any technique based on creating destructive interference is inherently effective only against interrogating signals within a narrow band.
Therefore, it is a primary object of this invention to provide an improved phased array antenna with a reduced peak and average RCS as compared to known phased array antennas.
Another object of this invention is to provide an improved phased array antenna as just mentioned, such antenna further maintaining its lower RCS over a broad band of frequencies.
These and other objects of this invention are attained generally by providing a phased array antenna in which: (a) the radiation elements and other beam forming elements of such array are disposed on a curved suface, thereby to impress a tapered phase distribution on energy reflected from such radiating and beam forming elements; and, (b) a wire mesh is used to shape the beam of the phased array antenna in the plane orthogonal to the plane of the array's radiation elements, the spacing of the wires making up such grid being as wide as possible and yet to close enough to retain the desired performance of the antenna.